The present invention generally relates to an image signal processing method and an apparatus of reducing color noises especially of a NTSC (National Television System Committee) video or the like in a video, a movie camera, a video printer and so on handing color images.
In recent years, highly faithful images can be reproduced with the use of printing image technology such as sublimation type heat transfer system or the like as the development of a hard copy technique, especially the hard copy technique of a full color. In the color reproduction, the reproducing capability equal to the silver salt photograph is provided by the recording material and the image processing. Even in the point of the resolution, it is approaching to the silver salt photograph with the use of the highly detailed image signals.
In the video printer for recording the television signals of the existing system, sufficient resolution cannot be obtained with respect to the resolution of the printer, because the resolution is restricted from the zone limit of the image signals of the NTSC or the like. Especially, in the color signal (color difference signal), the present situation is that the images blotted much in color are recorded, because the horizontal resolution is as extremely low as several tens in the resolution.
FIG. 9 shows a principle of removing the image noises in the first conventional embodiment. In FIG. 9, the brilliance signals 901 are inputted into a high pass filter (HPF) 90 and a low pass filter (LPF) 91 and are respectively separated into high frequency component 902 and the low frequency component 903 so as to reduce the noises with respect to the brilliance signal 901 separated from the image signal. The high frequency component separated by the HPF90 is non-linearly processed by a non-linear processing circuit 92 so as to remove the noise components within a constant amplitude range for obtaining a signal as shown in reference numeral 904. The signal is smoothed by an integration circuit 93, is corrected so that it may become the same in amplitude as before the input by the gain correcting circuit 94, and becomes a signal shown in reference numeral 905. The low frequency component separated by the LPF71 is added with an adder 95 to the high-pass brilliance signal with the noises of the high frequency component being removed as described hereinabove, the brilliance signal 906 with only the noises thereof being removed is outputted (Television Technique, Extra Edition 39 to 40p, September 1986).
An image appliance (Japanese Patent Publication 3-207192) with the color noises being reduced is provided as the second conventional embodiment. FIG. 10 shows a digital data processing means for removing the color noises by the image appliance in the second conventional embodiment. In FIG. 10, reference characters D-8 through D8 are continuous image data (color difference data) in the vertical direction with respect to the scanning lines of television and so on, reference numerals 1001 through 1017 are conversion table RAM so as to effect a computing operation shown in an (arithmetic equation 1) with respect to the input color difference data D-8 through D8. ##EQU1## wherein a computing value, with respect to each color difference data, of 8 bits is accommodated in the conversion table RAM 1001 through 1017 (for example, the values of a-8*255 through a-8*0 are accommodated in the conversion table RAM801), ai is defined by an (arithmetical equation 2). ##EQU2##
The (arithmetical equation 2) uses a filter of the characteristics of becoming a cut-off frequency .pi./4 when the sampling frequency has been made 2.pi..
Therefore, the output color difference data D0' becomes the low pass filter applied in the vertical direction of the image, with the color noises in the vertical direction of the image being reduced.
When the image noise removing operation in the above described first is adapted to the color signal, the frequency of the noise becomes a low frequency component, because the frequency zone of the color signal is narrower with respect to the frequency zone of the brilliance signal. The low-pass filter which is extremely low in the cut off frequency has to be used, with a problem that the resolution of the color is considerably deteriorated by the low-pass filter. The image signal recorded on the VTR is considerably large, as compared with the brilliance, in the noise increase of the color signal.
In the image appliance in the above described second conventional embodiment, the smoothing filtering in the vertical direction is applied with respect to the color signal so as to reduce the color noises in the image vertical direction and has a large effect with respect to the high frequency noises of the color in a portion where an even color tone continues. The color signal changes are often sudden with respect to the edge portion (for example, contours) sudden in the brilliance change in the vertical direction of the image. In the image appliance of the second conventional embodiment, as the even smoothing filtering is applied in the vertical direction as described hereinabove in the image appliance of the second conventional embodiment, the smoothing operation is effected even with respect to the edge portion where the change in the color signal is larger. Therefore, color blots are caused in the vertical direction of the contours of the image, with a problem that the resolution of the color signal is deteriorated. The picture elements to be referred to in the filtering are increased (refer to 17 picture elements in the second conventional embodiment) so as to improve the effect of the filtering, and further bad influences are given to the color blots in the vertical direction of the contours of the image, with a problem that increased circuit scale is provided.